Conventionally, class-A amplifiers (class-A output stages) are known to always carry significant current level to the output device(s). Thus they typically have a large quiescent current, and are often inefficient in terms of power. The quiescent current may be defined as the current level in the amplifier when it is producing an output of zero. Class-A amplifiers typically vary a large quiescent current in order to generate a varying current in the load. More efficient class-A amplifiers make use of what is commonly known as a double ended or push-pull arrangement, where a pair of transistors is connecting to the two rails. One of them is an NPN bipolar transistor, the other is a PNP bipolar transistor. Ideally, these two transistors have equivalent properties except for the difference in the signs of their voltages and currents. However, even in a push-pull arrangement power losses due to the quiescent current are an issue.
Class-B amplifiers are also often used in a push-pull arrangement connected to the two rails, while only one of the two bipolar transistors is conducting at a given time instance. Because one of the bipolar transistors is switched on and the other one is switched off the dissipation of power is lower. In quiescent condition, both bipolar transistors are turned off so that no quiescent current is flowing. However, push-pull type class-B amplifiers tend to have difficulty whenever the changing polarity, where one transistor is to be switched off and the other one is to be turned on. The result is what is called crossover distortion, which is often is enhanced due to non-linearity in the transistors. An exemplary discussion of class-B amplifiers can be found for example in P. J. Walker, “Current dumping audio amplifier”, Wireless World, 1975, 81, pp. 560-562. Class-AB amplifiers typically use a push-pull arrangement and may be considered a hybrid of the class-A amplifiers and Class-B amplifiers in that they operate similar to Class-B amplifiers, but in contrast to them both transistors conduct current at the same time. Hence, even when the output current is to be majorly provided from one of the two transistors, the other one is still conducting a small current. This allows significantly reducing or even eliminating crossover distortion, but the exact choice of quiescent current has major impact on the level of distortion generated in the class-AB amplifier. Often, the bias voltage applied to the base of the bipolar transistors to set this quiescent current has to be adjusted with the temperature of the output transistors, which typically requires the class-AB control circuitry more complex, e.g. due to requiring an exact copy of the voltages at the bipolar transistors for setting the quiescent current. An example of a class-AB control circuit is for example known from D. Monticelli, “A Quad CMOS Single-Supply Opamp with Rail-to-Rail Output Swing”, ISSCC Dig. Tech. Papers, pp. 18-19, February 1986.
In high-voltage applications, where the rail voltages are several tens of volts and the currents to be delivered by the output stage may be several hundreds of milliamps (mA) or more, such amplifier stages may be very difficult to realize on-chip, i.e. on an integrated circuit. For example, conventional class-AB amplifiers require a large die area since transistors capable of handling high voltage and currents are large. Moreover, the on-chip power dissipation necessitates a good heat sink, which is typically large in size and costly.
Sometimes, on-chip class-D amplifiers using MOS transistors and providing an output signal based on pulse-width-modulated signals (PWM) are therefore used in high-voltage, high-current applications. However, due to switching (PWM), class-D amplifiers may cause interference with other analogue circuitry present in the system, which may be undesirable. Another solution for providing a high-voltage, high-current amplifier may be a fully discrete solution on a circuit board, but this would require a large board area and component count.